Machines with high precision positioning mechanisms are used in many industries for accurately measuring the tolerance of machine parts and other components as well as for positioning tools for performing highly precise operations on parts and components. Typically, a tool or a touch sensitive probe for position measurement is mounted on the end of a support structure that allows the probe or tool to be moved in three dimensions by selective translation along three orthogonal axes.
Gantry-type support structures are commonly used in such machines, particularly coordinate measuring machines. Such a gantry-type structure typically includes a base, a table resting on the base, a gantry structure which rides on parallel, spaced rails supported by the base and a carriage which rides on the gantry structure. A vertically movable element, typically referred to a Z-rail, includes a touch sensitive probe disposed on the lower end thereof, and the Z-rail moves vertically with respect to the carriage, while the carriage moves horizontally along a rail disposed on the gantry structure. Gas bearings are used to facilitate the movement of the gantry structure on its associated rails. Typically, the gantry structure is driven only along one rail. The probe disposed on the end of the Z-rail therefore can be moved in three dimensions to be positioned at any point on the table for measuring a position on a part to be tested.
Electronic sensors are provided on each rail for sensing the position of the probe or tool in terms of its X, Y and Z coordinates. Typically, a microcomputer is provided within the device which causes the probe to be moved through a series of specified X, Y and Z locations for a specified period of time to either measure or work on the part resting on the table.
In use, the gantry-type structure is rapidly accelerated and decelerated to deliver the probe or tool to the point at which it is to perform its work or measurement. Such rapid movements are necessary so that measurements can be taken rapidly and so that the period for testing a part is not intolerably long. In part because such gantry-type structures are generally driven only along one rail, vibrations can result in the structure because of the structure's inherent inertia. A torque is produced about the point at which the drive is coupled to the gantry-type structure, and it may be assumed for most purposes that the torque is applied at the center of gravity of the gantry-type structure and has a moment arm which extends from the point of coupling of the drive to the structure to the center of gravity thereof. Since the gantry-type structure has a certain elasticity, and since continued movement of the structure is resisted by the drive, and by the bearings which maintain the proper positioning of the structure, this torque causes vibrations to be set up in the gantry about the point at which the gantry is coupled to the drive. These vibrations are then transferred to the bearings, which can act like a spring. Obviously, it is desirable to damp the vibrations out of the system as quickly as possible to avoid measurement errors. For high precision coordinate measuring machines, it is essential that the measurements be taken when no vibrations are present, so that the precise alignment can be maintained. The time required for the vibrations in the support structure to damp out of the system is termed the settling time.
Certain characteristics of a gantry-type positioning machine have a substantial effect on the amplitude and frequency of the vibrations in the structure. Very soft or dynamically marginal air bearings tend to increase the amplitude of vibration and settling time of the system. Also, if the bearings are unstable, particularly at a resonant frequency of the gantry-type structure, the settling time will be increased. The amplitude of the vibrations and the length of the settling time also depend on the stiffness of the gantry structure itself, as well as on the distance of the center of mass of the gantry from the X axis drive. The amplitude of the vibrations will increase as the carriage is moved along the Y-rail away from the X axis drive or as the Z-rail is moved away from the X axis drive. This increase in amplitude occurs because, as the distance between the center of mass of the gantry and the X axis drive increases, the moment arm of the force and thus the applied torque increases.
Therefore, it is an object of the present invention to reduce vibrations in a gantry-type machine having a high precision positioning mechanism.
It is another object of the present invention to reduce vibrations in a coordinate measuring machine.
It is a further object of the present invention to provide a vibration damper for a coordinate measuring machine.
It is another further object of the present invention to reduce the settling time of vibrations in a gantry-type structure which are induced by acceleration and deceleration of the machine components during rapid measurements.
It is yet another object of the present invention to improve the throughput of a coordinate measuring machine.